A water supply sub-system for connection to a water quality analyzer unit includes a water purification unit co-located with the water quality analyzer unit. Raw reference water is introduced into the water purification unit and passed along a flow path including at least a water purification element including a reverse osmosis unit. An automated reference water valve provides purified reference water to the water quality analyzer unit at predetermined time intervals for use in instrumental zeroing events. Low volume but high efficiency water purification elements are used advantageously to purify small volumes of water at regular intervals, thereby eliminating the need to maintain a reservoir of water for instrumental zeroing purposes.